David Galt

Characterization of a tunable thin film microwave YBa2Cu3O7−x/SrTiO3 coplanar capacitor

We have fabricated and characterized electrically tunable high temperature superconductor coplanar microstrip resonators incorporating tunable SrTiO3 ferroelectric thin films. The low frequency capacitance of the SrTiO3 capacitor is measured directly. High frequency capacitance and loss information are extracted from the observed resonances and compared with the low frequency data. Hysteresis loops display an onset of ferroelectricity at 160 K. The spontaneous charge and coercive voltage (at 10 kHz) as a function of temperature are extracted from these loops.

Improved low frequency and microwave dielectric response in strontium titanate thin films grown by pulsed laser ablation

We have grown epitaxial strontium titanate films on lanthanum aluminate substrates at a range of oxygen pressures and substrate temperatures. The complex dielectric function was measured as a function of temperature and electric field bias using a microwave ring resonator and a flip-chip technique. The films having the highest dielectric constant were grown with an oxygen pressure of 600 mTorr and showed large grains in the plane of the film. The small-signal dielectric constant of these films could be changed by a factor of 4 by applying an electric field. The films with the highest dielectric constant showed increased losses, but an improved figure of merit for application to tunable circuits.

Ferroelectric thin film characterization using superconducting microstrip resonators

We describe a novel technique for characterizing the dielectric response of ferroelectric thin films at microwave frequencies. The method involves a microstrip resonator which incorporates a ferroelectric capacitor at its center. To demonstrate this method rye have fabricated a superconducting microstrip resonator from a laser-ablated YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) film on a LaAlO/sub 3/ (LAO) substrate with a SrTiO/sub 3/ (STO) capacitor at its center. We report the observed dielectric behavior of the STO laser ablated film as a function of bias at liquid He and N/sub 2/ temperatures and at high and low frequencies. It is observed that the electrically tunable dielectric constant of the STO film is roughly independent of frequency up to 20 GHz (especially at high bias). The loss tangent of the STO/LAO capacitor decreases with increasing bias and is apparently independent of frequency between 6 and 20 GHz.<<ETX>>

Oxide superconductors and ferroelectrics—Materials for a new generation of tunable microwave devices

We describe materials deposition and characterization for a broad class of tunable microwave devices using high-temperature oxide superconductor and voltage-tunable oxide ferroelectric thin-film multilayer structures. Tl-Ba-Ca-Cu-O thin films deposited by sputtering ore-beam evaporation were patterned into microwave resonators, each consisting of two colinear microstrip line sections separated by a 5–20μm gap. A Ba0.1Sr0.9TiO3 (BST) layer was then over-coated to fill the gap. The relative dielectric constant of the BST films deposited by physical vapor or chemical techniques was measured at 77 K in the 1–10 MHz range and found to vary by up to a factor of 2 or more with voltage bias in test capacitor structures using Pt electrodes. In the BST (variable capacitor)-HTSC microwave resonator structures, the change of the relative dielectric constant of the BST under voltage bias has allowed variation of the fundamental frequency of up to 80 MHz in the 5–10 GHz range at 4 K. Film deposition by various techniques and associated structural, morphological, and electronic properties, as well as materials compatibility issues, are discussed.

Some microwave applications of BaSrTiO3 and high temperature superconductors

Abstract The group at the University of Colorado has been working on the applications of BaSrTiO3 and high temperature superconductors to microwave circuits and systems with support from Superconducting Core Technologies, NASA and ONR. This work has included the development of a voltage tunable oscillator, a phase shifter, and a voltage tunable phased array antenna system. An important aspect of this program is the lattice match between the high temperature superconductor and the ferroelectric materials which enables us to grow thin films of either material on each other. By choosing the appropriate barium stronium ratio we are able to obtain changes in the dielectric constant of 5 to 1 in bulk capacitors at 77 K, the liquid nitrogen boiling temperature. Phase shifts of a little less than 126° at 10 Ghz have been obtained by applying 5 volts on a one centimeter coplanar line. The proposed phased array systems should enable us to point the beam radiated from the antenna in an arbitrary direction using two ...

Growth and characterization of YBCO/insulator/YBCO trilayers

Multilevel circuits for high-frequency applications of high-T/sub c/ superconductors require low-dielectric-constant insulators between superconducting layers. Initial studies of CeO/sub 2/ thin films as the insulating layer in YBCO/insulator/YBCO structures revealed insufficient isolation between YBCO layers. Trilayer structures employing thin-film composite dielectrics of CeO/sub 2/ and SrTiO/sub 3/ were therefore investigated. Each layer grows epitaxially with a morphology comparable to that of a single YBCO film. Transport critical current density measurements of the top YBCO layer resulted in J/sub c/=2*10/sup 5/ A/cm/sup 2/ at 77 K, a factor of 10 lower than for single films. Trilayer structures had a microwave surface resistance at 10 GHz and 4 K of 50 mu Omega , comparable to that of single films. Preliminary low-temperature measurements of the dielectric constant of composite insulator structures gave values an order of magnitude lower than for pure SrTiO/sub 3/.<<ETX>>

Tunable high temperature superconductor microstrip resonators

Electrically tunable high-temperature superconductor microstrip resonators incorporating YBa/sub 2/Cu/sub 3/O/sub 7-x/ superconductor and SrTiO/sub 3/ ferroelectric films have been fabricated and characterized. The resonators consist of two collinear microstrip line sections separated by a 5- mu m gap. The capacitance of the gap influences the frequencies of the odd-order coupled resonances. Inductively choked DC bias lines are attached to each line section so that a bias voltage can be applied to the gap. When the gap is filled with a ferroelectric material, the odd resonances can be tuned. Frequency shifts of 300 MHz have been observed with a bias voltage of 50 V for resonances at 5.6 GHz and 11.6 GHz. The tunability is independent of temperature from 4 K to 80 K. An upper bound for the loss tangent of the SrTiO/sub 3/ capacitor is extracted from the resonance Q, and tan delta <0.07 at 4 K is found.<<ETX>>

1.3-μm VCSEL production issues

1.3 μm VCSELs have been under development for several years. In this work, we discuss several requirements and characteristics that allow a device to be manufacturable in high volume with excellent yield.

Performance of monolithic 1.3 μm VCSELs in telecom applications

This paper reviews the recent advances made in monolithic GaAs based, directly modulated, 1.3micrometers VCSEL array technology. Such VCSEL arrays are poised to begin occupying a large telecommunications application space. We present data demonstrating 1.3 micrometers VCSELs having ~ 1mW optical power across a wide temperature range of 10 to 90 degree(s)C while operating with low voltages of less than 2.5V. The data includes performance on typical 8 and 12 element arrays at the die level as well in the module. We also present very encouraging preliminary reliability results.

Reliability of 1.3 micron VCSELs for metro area networks

Vertical Cavity Surface Emitting Lasers (VCSELs) have been widely adopted in the 850nm data communications markets with great success. Using this technology as a basis, we have developed a 1.3 μm InGaAsN VCSEL and VCSEL Array technology for telecommunications applications. Since the reliability requirement of this market is less than 150 FITs over 20 years, we focused a great deal of development time on the reliability of the device, and so far have been able to predict an MTTF of over 13 million hours or 71 FITs. This report provides a brief summary of the characteristics of the VCSEL in various stress conditions and the methodology used to measure both the wear-out and random failure rates of the devices.